Not telling the insurance company about modifications, having a bump and not getting paid out for upgraded parts is one thing, they probably aren't too interested in remaps and turbos then.
The day you run someone over and kill them I can assure you that so many people will be looking for a reason to throw the book at you modifications to the car will be discovered. If the insurance company is not aware I would expect big trouble! Its also important that all modifications are installed correctly and are safe on the car for exactly the same reasons.